Fixed tangent calculation for open tubes
Fixed tangent calculation for open tubes

Without this correction, the tangent calculation wraps
around the points array, giving spurious results at both
ends.

hronoses/vispy,michaelaye/vispy,jdreaver/vispy,ghisvail/vispy,Eric89GXL/vispy,kkuunnddaannkk/vispy,inclement/vispy,RebeccaWPerry/vispy,hronoses/vispy,sh4wn/vispy,dchilds7/Deysha-Star-Formation,drufat/vispy,dchilds7/Deysha-Star-Formation,sbtlaarzc/vispy,julienr/vispy,jay3sh/vispy,ghisvail/vispy,jdreaver/vispy,drufat/vispy,inclement/vispy,Eric89GXL/vispy,QuLogic/vispy,bollu/vispy,srinathv/vispy,bollu/vispy,julienr/vispy,michaelaye/vispy,jdreaver/vispy,RebeccaWPerry/vispy,sbtlaarzc/vispy,hronoses/vispy,ghisvail/vispy,inclement/vispy,michaelaye/vispy,sh4wn/vispy,jay3sh/vispy,QuLogic/vispy,kkuunnddaannkk/vispy,srinathv/vispy,Eric89GXL/vispy,dchilds7/Deysha-Star-Formation,jay3sh/vispy,sbtlaarzc/vispy,kkuunnddaannkk/vispy,srinathv/vispy,sh4wn/vispy,julienr/vispy,RebeccaWPerry/vispy,drufat/vispy,bollu/vispy,QuLogic/vispy